This code
library(ggplot2)
ggplot(mtcars, aes(mpg, hp)) +
facet_wrap(am ~ cyl, ncol = 6) +
geom_point()
produces the following graph
The variable "am" has two values (0 and 1) that are repeated 3 times each. I wonder whether it is possible to include a single strip with the value 0 centered (and the same for the value 1).